The present invention relates to a fixing device for an image forming apparatus for fixing an image formed on a recording material surface and an image forming apparatus using the fixing device. More specifically, the present invention relates to a fixing device for heat-fixing a yet unfixed image formed on a recording material by passing and conveying the recording material between a pair of a fixation roller and a pressure roller under pressure, and an image forming apparatus using the fixing device, particularly a color image forming apparatus comprising a heat fixing device and a plurality of image forming means.
Heat fixing devices have been widely used as means for fixing a yet unfixed image formed through an electrophotographic process, i.e., a yet unfixed toner image formed on a surface of a recording material such as paper.
Image forming apparatus using such fixing devices includes color image forming apparatus as shown in FIGS. 3 and 4 wherein image formation is performed by using four color toners.
Referring to FIG. 3, the color image forming apparatus comprises a color laser beam printer M using four drums provided with four independent light scanning means for four colors, respectively. More specifically, the laser beam printer M includes four image forming means (image forming stations) Pa, Pb, Pc and Pd for four colors disposed in succession. Each image forming station includes a drum-shaped electrophotographic photosensitive member as a latent image-bearing member (hereinafter, referred to as xe2x80x9cphotosensitive drumxe2x80x9d), and a developing device using a prescribed color toner and a transfer device integrally disposed around a peripheral surface of the photosensitive drum.
In each of the (four) image forming stations, an image formed on the photosensitive drum is transferred onto a recording material such as paper carried on a conveyance means. The conveyance means is disposed along the respective (four) image forming stations, so that the respective color toner images formed on the corresponding photosensitive drums are successively transferred on the recording material carried on the conveyance means to the fixing device where the respective color toner images (yet unfixed images) are heat-fixed at the same time.
FIG. 4 is an enlarged sectional view of the four image forming stations described above.
Referring to FIG. 4, the four image forming stations Pa, Pb, Pc and Pd are used for forming toner images of magenta, cyan, yellow and black, respectively.
Each image forming station Pa (Pb, Pc, Pd) includes a rotatable photosensitive drum 1a (1b, 1c, 1d), and other members including a charger 12a (12b, 12c, 12d), a developing device 2a (2b, 2c, 2d) for supplying a corresponding color toner, and a cleaner 4a (4b, 4c, 4d) successively disposed along the rotation direction of the photosensitive drum 1a (1b, 1c, 1c). On a downstream side of the respective developing devices 2a-2d, i.e., below the photosensitive drums 1a-1d on FIG. 4, a transfer unit 3 is disposed. The transfer unit 3 includes a transfer belt 31 carrying thereon the recording material used common to the respective image forming stations Pa-Pd and four transfer chargers 3a, 3b, 3c and 3d disposed opposite to the photosensitive drums 1a, 1b and 1d, respectively.
In the above-mentioned color laser beam printer, paper P (as the recording material) supplied from a paper supply cassette 61 is conveyed to the respective image forming stations Pa-Pd while being supported on the transfer belt 31. Thereafter, the respective color toner images formed on the respective photosensitive drums 1a-1d are successively transferred onto the paper P carried on the transfer belt 31. After the succession of transfer steps for the respective color toner images is completed, the paper P having thereon yet unfixed color toner images is separated from the transfer belt 31 to be conveyed to the fixing device (fixing unit) 5 by a conveyance belt 62 (shown in FIG. 3) as a recording material-guiding means.
FIG. 5 is a schematic sectional view of an embodiment of a conventional heat fixing device (as described, e.g., in Japanese Laid-Open Application (JP-A) No. 7-219378) as the fixing device (unit) 5 shown in FIG. 3.
Referring to FIG. 5, the heat fixing device includes a pair of rotatably disposed rollers, i.e., a fixation roller 51 and a pressure roller 52 pressing the fixation roller 51 via the paper P (recording material) carrying thereon a yet unfixed image, and further includes a release agent application means 53 as a means for supplying and applying a release agent onto the surface of the fixation roller 51 and roller cleaning means 54 and 55. Inside the fixation roller 51 and the pressure roller 52, heaters 56 and 57 (e.g., halogen lamps) are disposed, respectively. Further, thermistors 58 and 59 are disposed in contact with the surfaces of the fixation roller 51 and the pressure roller 52, respectively, whereby voltages supplied to the heaters 56 are controlled through a temperature control circuit to adjust surface temperatures of the fixing roller 51 and the pressure roller 52 to desired temperatures.
The roller cleaning means 54 provided to the fixation roller 51 has a cleaning function of removing, e.g., toner particles detached from the surface of the paper P and attached to the fixation roller surface at the time of fixation. On the downstream side of the cleaning means 54 with respect to the rotation direction (indicated by an arrow) of fixation roller 51, the release agent application means 53 is provided to the fixation roller 51. After the cleaning (removal of toner particles), a release agent such as a silicone oil is applied onto the surface of the fixation roller 51 by the release agent application means 53. The release agent is used for facilitating the separation of the paper P (recording material) from the fixation roller 51 after the fixing step and preventing an occurrence of offset phenomenon of toner particles during the fixation.
The cleaning means 54 includes a cleaning web 54a comprising an elongated heat-resistant nonwoven fabric, a pressing roller 54b for pressing the cleaning web 54a against the fixation roller 51, a feeding roller 54c for feeding a fresh cleaning web 54a, and a take-up roller for gradually winding up the cleaning web 54a lowered in cleaning performance due to the attachment of toner particles. Particularly, in order to prevent an occurrence of detection failure of the thermistor 58 caused by the attachment of toner particles detached from the paper surface to the thermistor used for monitoring the surface temperature of the fixation roller 51, the cleaning means 54 is disposed on the upstream side of the fixation roller 51 with respect to the rotation direction thereof.
In the cleaning means 54 shown in FIG. 5, the cleaning web is wound up in such a manner that a prescribed amount of the cleaning web 54a is wound up in a direction opposite to the rotation direction of the fixation roller 51 by energizing a solenoid (not shown) to actuate a one-way clutch when a prescribed number of sheets of paper P is counted by a counting means (counter) provided to the image forming apparatus (laser beam printer). The take-up operation in the direction opposite to the fixation roller rotation direction prevents the cleaning web 54a from being wound up towards the fixation roller side by the rotation of the fixation roller 51.
On the other hand, the release agent application means 53 includes an oil tank (container) 53a containing therein an release agent such as a silicone oil, conveyance rollers 53b and 53c for carrying and supplying the release agent from the oil tank 53a, an application roller 53d for applying the release agent supplied via the conveyance rollers 53b and 53c onto the surface of the fixation roller 51, and a regulation blade 53e for regulating an amount of the release agent applied onto the surface of the application roller 53d. Particularly, in order to ensure a uniform application of the release agent (e.g., silicone oil) onto the fixation roller surface, the release agent application means 53 is disposed on the downstream side with respect to the rotation direction of the fixation roller 51. As a result, the release agent once applied onto the fixation roller surface is not removed by the thermistor 58 contacting the fixation roller 51.
The application roller 53d may, e.g., be formed of a sponge rubber coated with a silicone rubber layer in a roller shape and is rotatable together with the fixation roller 51. The release agent present on the surface of the application roller 53d is transferred onto the fixation roller 51 during the rotation of the application roller 53d while abutting the fixation roller 51, thus effecting application of the release agent onto the fixation roller 51.
The regulation blade 53e may generally comprise an elastic blade of, e.g., fluorine-containing rubber and is disposed fixedly in abutment with the application roller 53d. An excessive amount of the release agent supplied via the conveyance rollers 53b and 53c is removed from the surface of the application roller 53d by abutting the regulation blade 53e against the application roller 53d. At the time of abutment of the regulation blade 53e with the application roller 53d, an amount of the release agent present on the application roller surface is effectively regulated so as not to be outside an appropriate range.
The pressure roller 52 is provided with the cleaning means 55 having the same mechanism as the cleaning means 54 for the fixation roller 51. Specifically, the cleaning means 55 includes a cleaning web 55a, a pressing roller 55b, a feeding roller 55c, and a take-up roller 55d. Although the pressure roller 52 do not directly contact the yet unfixed toner, a part of the toner detached from the recording material can be attached onto the pressure roller 52 in some cases. The thus attached toner to the pressure roller 52 is removed by the cleaning means 55.
The release agent applied onto the fixation roller 51 is also attached onto the surface of the pressure roller 52 during the abutment with the fixation roller 51. In order to remove the thus excessive amount of release agent present on the pressure roller surface, the pressure roller 52 is provided with a removal blade 60 for removing the release agent disposed in abutment with the pressure roller 52. If the removal blade 60 is not provided to the pressure roller 52, the excessive amount of release agent accumulated on the pressure roller surface remains at a nip between the pressure roller 52 and the fixation roller 51 where the pressure roller 52 abuts the fixation roller 51 via the recording material. As a result, the release agent can, e.g., be attached onto the surface of the recording material to result in stains in some cases. In the case where a transparent laminate film for overhead projector (OHP film) having a very small friction coefficient compared with paper is used as the recording material, the OHP film is liable to slip to cause an occurrence of conveyance failure to the nip formed between the pair of (fixation and pressure) rollers 51 and 52.
The removal blade 60 is formed of, e.g., a silicone rubber or a fluorine-containing rubber, and is disposed in abutment with the pressure roller 52 with an appropriate pressing depth in a forward or reverse direction with respect to the rotation direction of the pressure roller 52.
When the recording material (e.g., paper P) is conveyed to the above-described fixing device, the pair of fixation roller 51 and pressure roller 52 is rotated and a release agent such as a silicone oil is applied onto the surface of fixation roller 51. The recording material is subjected to heating and pressing at substantially constant temperature and pressure from both sides thereof at the time of passing between the pair of rotating rollers 51 and 52. As a result, a yet unfixed toner (image) is melted and fixed on the recording material to complete full-color image formation. The recording material subjected to fixation of the yet unfixed toner image is separated from the pressure roller 52 by a separation claw 68 to be sent to the outside of the image forming apparatus.
In the above-mentioned conventional image forming apparatus using the heat fixing device, when a relatively lower process speed (image forming rate) of, e.g., ca. 10 sheets/min. for paper (or at a further lower process speed for OHP film) is employed, it is possible to effect a smooth heat fixation and a subsequent discharge operation with no fixation failure.
However, it has been found that the above-mentioned heat fixing device is liable to be accompanied with the following difficulties when the fixing device is used as it is in a high-speed image forming apparatus.
Specifically, for example, when a full-color (toner) image is formed on paper at a process speed of ca. 10 sheets/min., it is possible to form the full-color image with no difficulty even if an amount of a release agent (e.g., silicone oil) applied onto the fixation roller is at most 10 mg/A4-size sheet.
On the other hand, if the number of (image forming) sheets formed per 1 min. is further increased (i.e., a process speed of not less than 10 sheets/min.), the sheets of recording material (paper in this case) is conveyed at shorter intervals. In this case, unless an amount of a release agent applied onto the fixation roller is increased, separation failure of recording material from a roller after fixation is liable to occur. In order to prevent the separation failure, it has been found that an amount of the recording material should be regulated in a range of ca. 20-30 mg/A4-sized sheet.
However, in the above-mentioned conventional fixing device as shown in FIG. 5, it was difficult to stably regulate an appropriate amount (20-30 mg/A4-sized sheet) of the applied release agent by using the release agent application means 53. In some cases, the application amount of release agent exceeded 30 mg/A4-sized sheet or the release agent was localized, whereby the release agent such as silicone oil was attached onto the surface of the recording material in a streak form, thus staining the recording material (i.e., occurrence of streak-shaped oil).
The amount of release agent applied onto the fixation roller 51 is directly proportional to the amount of release agent present on the application roller 53d abutting the fixation roller 51, so that the application amount of release agent on the fixation roller 51 is controlled by the regulation blade 53e for regulating an amount of release agent applied to the application roller 53d. More specifically, the amount of release agent (applied to the application roller 53d) is regulated by appropriately setting abutting conditions (such as abutting angle and abutting pressure) of the regulation blade 53e with respect to the application roller 53d or appropriately changing an edge shape of the regulation blade 53e. For example, when the edge shape is changed to be acute, an amount of removed release agent is increased to lower the application amount of release agent to the fixation roller 51. On the other hand, when the edge shape is changed to be round, the removal amount of release agent is decreased to increase the release agent application amount.
In the case where the regulation blade 53d having an edge shape as shown in FIG. 5 was used, the amount of release agent applied to the application roller 53d could readily be set to be a smaller amount of, e.g., at most 10 mg/A4-sized sheet but it was difficult to stably regulate the release agent application amount in a range of 20-30 mg/A4-sized sheet. Even when the application amount was once regulated in the range of 20-30 mg/A4-sized sheet, the regulated amount was liable to became unstable, thus causing such a phenomenon that the applied release agent (such as silicone oil) slipped through the abutting portion with the application roller 53d. As a result, the slipped release agent was attached onto the recording material (particularly on the OHP film) in the form of streaks at the time of fixation, thus lowering a resultant image quality.
In order to solve the above problem such that the regulated amount of release agent is liable to become ununiform, it is effective to utilize a regulation blade 53e having a chamfered or beveled abutting edge against the application roller 53d. Examples of such a chamfered regulation blade 53e may include one described in JP-A 7-295422.
By using such a chamfered regulation blade, when compared with the regulation blade as shown in FIG. 5, it becomes possible to stably control the regulated amount of release agent (e.g., silicon oil) but has been found to be accompanied with the following difficulty.
By using the chamfered regulation blade, the amount of release agent present on the application roller surface is considerably increased when compared with the case of using the regulation blade having a rectangular edge (90 degree-edge) as shown in FIG. 5. However, under some abutting conditions, a chamfered regulation blade 53e disposed, e.g., as shown in FIG. 6 causes a phenomenon such that the release agent slipped through the abutting portion (edge T2) of the regulation blade 53e with the application roller 53d is gradually accumulated on a surface A of the regulation blade. When the amount of the accumulated release agent at the surface A is increased, the excessively accumulated release agent is dropped on the surface of the application roller 53d. The resultant drops of the release agent are applied onto the fixation roller 51 as they are, thus leading to attachment of the release agent (e.g., silicone oil) in the form of drops onto the image region of the recording material. The attached drop-like release agent remains as stains on the image region, thus causing image failure.
A principal object of the present invention is to provide a fixing device and an image forming apparatus having solved the above-mentioned problems.
A specific object of the present invention is provide a fixing device of a heat fixation scheme capable of performing a smooth fixation operation and a subsequent discharge operation particularly with no fixation failure even when used in a high-speed image forming apparatus.
Another object of the present invention is to provide a fixing device for an image forming apparatus capable of suppressing a ununiformity in application amount of a release agent such as a silicone oil and preventing an occurrence of image failure due to dropping of the release agent onto an image region of a recording material, such as paper or OHP film, particularly OHP film.
A further object of the present invention is to provide an image forming apparatus including the above-mentioned fixing device of heat fixation scheme.
According to the present invention, there is provided a fixing device for an image forming apparatus for heat-fixing a yet unfixed image formed on a recording material by passing and conveying the recording material between a pair of rollers, comprising:
a pair of a fixation roller and a pressure roller which are rotatably disposed in contact with each other under pressure, and
application means for applying a release agent onto the fixation roller, including at least an application roller disposed in contact with the fixation roller and a regulation blade for regulating an amount of the release agent applied onto the surface of the application roller; wherein
the regulation blade is disposed to abut the application roller so that an extension direction of the regulation blade towards the application roller is substantially counter to a circumferential moving direction of the application roller and is provided with a chamfer at a corner between major and minor surfaces of the regulation blade at a chamfer angle of 30-70 degrees between an extension direction of the major surface and the chamfer of the regulation blade, the regulation blade being disposed to abut the application roller at an abutting angle of 30-50 degrees between the major surface and a tangent line of the application roller at a position in abutment with the regulation blade.
By using the above release agent application means, it becomes possible to readily regulating an amount of the release agent to be applied onto the application roller (consequently the fixation roller) in an appropriate range, thus preventing ununiformity in application amount of release agent and image failure due to a dropping of release agent caused by an excessive amount of release agent slipped through an abutting portion (e.g., edge T2 shown in FIG. 6) of the regulation blade with the application roller.
In the present invention, the release agent may preferably be an oil, more preferably a silicone oil.
The present invention also provides an image forming apparatus including image forming means and the above-mentioned fixing device.
In this case, the image forming means may preferably be divided into a plurality of image forming means for forming images of a plurality of colors (e.g., magenta, cyan, yellow, black), respectively, thus allowing a full-color image formation.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.